fix(lsp): send didOpen if name changes on write (#19583)

`:saveas newName` changes the name of an existing buffer.
Due to the buffer re-use it skips the lsp attach phase and immediately
sends a `didSave` notification to the server.
Servers get confused about this, because they expect a `didOpen`
notification first.

Closes https://github.com/neovim/neovim/issues/18688

runtime/lua/vim/lsp.lua

@@ -371,7 +371,9 @@ do
       state_by_client[client.id] = state
     end
     if not state.buffers[bufnr] then
-      local buf_state = {}
+      local buf_state = {
+        name = api.nvim_buf_get_name(bufnr),
+      }
       state.buffers[bufnr] = buf_state
       if use_incremental_sync then
         buf_state.lines = nvim_buf_get_lines(bufnr, 0, -1, true)
@@ -381,6 +383,15 @@ do
     end
   end
 
+  ---@private
+  function changetracking._get_and_set_name(client, bufnr, name)
+    local state = state_by_client[client.id] or {}
+    local buf_state = (state.buffers or {})[bufnr]
+    local old_name = buf_state.name
+    buf_state.name = name
+    return old_name
+  end
+
   ---@private
   function changetracking.reset_buf(client, bufnr)
     changetracking.flush(client, bufnr)
@@ -1405,6 +1416,19 @@ local function text_document_did_save_handler(bufnr)
   local uri = vim.uri_from_bufnr(bufnr)
   local text = once(buf_get_full_text)
   for_each_buffer_client(bufnr, function(client)
+    local name = api.nvim_buf_get_name(bufnr)
+    local old_name = changetracking._get_and_set_name(client, bufnr, name)
+    if old_name and name ~= old_name then
+      client.notify('textDocument/didOpen', {
+        textDocument = {
+          version = 0,
+          uri = uri,
+          languageId = client.config.get_language_id(bufnr, vim.bo[bufnr].filetype),
+          text = buf_get_full_text(bufnr),
+        },
+      })
+      util.buf_versions[bufnr] = 0
+    end
     local save_capability = vim.tbl_get(client.server_capabilities, 'textDocumentSync', 'save')
     if save_capability then
       local included_text

test/functional/fixtures/fake-lsp-server.lua

@@ -67,10 +67,12 @@ local function expect_notification(method, params, ...)
   local message = read_message()
   assert_eq(method, message.method,
       ..., "expect_notification", "method")
+  if params then
     assert_eq(params, message.params,
         ..., "expect_notification", method, "params")
     assert_eq({jsonrpc = "2.0"; method=method, params=params}, message,
         ..., "expect_notification", "message")
+  end
 end
 
 local function expect_request(method, handler, ...)
@@ -257,6 +259,26 @@ function tests.basic_check_capabilities()
   }
 end
 
+function tests.text_document_save_did_open()
+  skeleton {
+    on_init = function()
+      return {
+        capabilities = {
+          textDocumentSync = {
+            save = true
+          }
+        }
+      }
+    end;
+    body = function()
+      notify('start')
+      expect_notification('textDocument/didOpen')
+      expect_notification('textDocument/didSave')
+      notify('shutdown')
+    end;
+  }
+end
+
 function tests.text_document_sync_save_bool()
   skeleton {
     on_init = function()

test/functional/plugin/lsp_spec.lua

@@ -535,6 +535,46 @@ describe('LSP', function()
       }
     end)
 
+    it('saveas sends didOpen if filename changed', function()
+      local expected_handlers = {
+        { NIL, {}, { method = 'shutdown', client_id = 1 } },
+        { NIL, {}, { method = 'start', client_id = 1 } },
+      }
+      local client
+      test_rpc_server({
+        test_name = 'text_document_save_did_open',
+        on_init = function(c)
+          client = c
+        end,
+        on_exit = function(code, signal)
+          eq(0, code, 'exit code')
+          eq(0, signal, 'exit signal')
+        end,
+        on_handler = function(err, result, ctx)
+          eq(table.remove(expected_handlers), { err, result, ctx }, 'expected handler')
+          if ctx.method == 'start' then
+            local tmpfile_old = helpers.tmpname()
+            local tmpfile_new = helpers.tmpname()
+            os.remove(tmpfile_new)
+            exec_lua(
+              [=[
+              local oldname, newname = ...
+              BUFFER = vim.api.nvim_get_current_buf()
+              vim.api.nvim_buf_set_name(BUFFER, oldname)
+              vim.api.nvim_buf_set_lines(BUFFER, 0, -1, true, {"help me"})
+              lsp.buf_attach_client(BUFFER, TEST_RPC_CLIENT_ID)
+              vim.api.nvim_buf_call(BUFFER, function() vim.cmd('saveas ' .. newname) end)
+            ]=],
+              tmpfile_old,
+              tmpfile_new
+            )
+          else
+            client.stop()
+          end
+        end,
+      })
+    end)
+
     it('BufWritePost sends didSave including text if server capability is set', function()
       local expected_handlers = {
         {NIL, {}, {method="shutdown", client_id=1}};